In passing through the human body, gamma photons have a certain probability of scattering due to the Compton effect. Such scattering changes the direction of motion of the photons and the energy of the photons. When a photon that has been scattered is recorded in the gamma camera image, false position information is derived from the scattered photons.
In principle, the scattered photons should be discarded. However, it is not easy to arrive at criteria that are efficient and effective for discarding scattered photons. For example, an energy level criterion is not effective because even though the photon loses part of its energy in the scattering process, the energy resolution of a typical gamma camera is such that there is a large amount of overlap in acceptable recorded energy between scattered and unscattered photons.
An object of the present invention is to both qualitatively and quantitatively improve the recorded images by significantly eliminating the contribution of the scattered photons to the final image and obtaining the practically Compton free images within seconds after acquisition. There are many prior art methods that also reduce, or attempt to reduce the contribution that the scattered photons make to the final image. However, of all the prior art methods and systems known, no high quality, practical method is known wherein the contribution of the scattered photons to the final image is significantly reduced and the final image is shown to be qualitatively and quantitatively practically free of Compton scatter contamination.
Some of the prior art methods are:
(a) Dual Window Acquisition
In addition to acquiring data using the standard, full energy window around the peak energy, a second image is acquired at "low" energy. The second image is then multiplied by an empirical constant and subtracted from the first image to reduce the effect of the scattered events on the final image.
(b) The Method of Axelsson et al (see the article "Subtraction of Compton-Scattered Photon Emission Computerized Tomography", J. Nuc. Med. 25, pp. 490-494, 1984)
This is a spatial deconvolution method which is designed to extract the exponential tail caused by Compton scattering from the point spread function. The magnitude of the exponential tail is considered a constant throughout the image.
(c) Weighted Acquisition Module
This method has been described in U.S. Pat. No. 4,780,823. Basically, what is described in the Patent is a constant spatial filter applied on the fly over the whole area of the image. The method does not remove the Compton scattered events at all, but tends to sharpen edges blurred by the scattering.
(d) The Method of Koral et al
This method is taught in U.S. Pat. No. 4,839,808. Therein the energy spectrum is fitted at each given location with a photopeak shaped curve combined with a third order polynomial that is assumed to represent the Compton scatter spectrum. The result of the fit is assumedly a separation of events at each point into a scattered and an unscattered portion. In order to obtain reasonable statistics and also save computation time, the energy spectra are in practice analyzed on a coarse spatial grid. The values of the Compton scatter fraction are then interpolated to obtain values for a finer imaging grid.
The basic shortcoming which all of the above methods have in common is that they do not utilize the innate physical properties of the energy spectrum to account for the Compton scattered events. As a result, the above Compton scatter correction methods all inherently cannot possibly correctly calculate and subtract Compton scattered events.
The methods (a) and (b) described above, are both totally empirical and non-local. As a result, neither method has been successful in overcoming artifacts due to Compton scattering and as a result, in practice, they are not used.
The weighted acquisition module method does not even attempt to separate the Compton scattered events from the unscattered events; rather, the method sharpens somewhat the edges blurred by the scattering. It does have limited use in qualitative images, but is practically totally useless in quantative SPECT imaging, which is expected to be a main application of scatter corrected imaging.
The method of Koral et al suffers from several drawbacks. The main drawback is the fact that the third order polynomial used for fitting the Compton scatter distribution does not truly define the physical distribution of the Compton scatter events. The third order polynomial is unrestricted especially at the high energy Compton scatter thresholds of the spectrum which overlap the photopeak. The high energy portion of the overlap is the most relevant area in which it is difficult to eliminate Compton scatter by conventional methods of energy windows.
With the fit being performed thousands of times per image, it is practically impossible to prevent using many incorrect composite curves which result in erroneous image quantitation and artifacts.